Method of making comparator color wheels

ABSTRACT

A method of depositing increments of colored resin in a groove formed in a groove formed in a surface of a transparent body, so that a color strip is formed in which at least one of three color properties, hue, value and chroma varies continuously along the length of the groove.

O United States Patent 1 1 1111 3,764,646 Hach Oct. 9, 1973 [54] METHODOF MAKING COMPARATOR 2,985,556 5/1961 Rowland 264/245 X COLOR WHEELS3,513,060 5/1970 Krystofi'm. 264/245 X 1,304,083 5/1919 Moore 264/246 X[75] Inven or: Cl ffor Bach, A I 3,678,141 7/1972 Metcalfe et a] 264/245x Assigneez Bach Chemical p y, i 3,354,025 11/1967 Aykaman et al 264/245X Iowa OTHER PUBLICATIONS [22] Filed: Apr. 22, 1970 Randolph et al.,Plastics Engineering Handbook, Rein- 1 pp No 38,635 hold, New York(1960), page 501.

Related U.S. Application Data Primary ExaminerRobert F. White [62]Division of Ser. NO. 526,532, Feb. 10, 1966, Pat. No. A i n Examin r-W-E. Hoag 3,520,626. Att0meyWolfe, Hubbard, Leydig, Voit & Osann,

Ltd. {52] U.S. Cl 264/162, 264/154, 264/246, 264/251, 264/267 [51] Int.Cl. B29c 21/00 [57] ABSTRACT [58 Field of Search 264/241, 245, 246, Amethod of depositing increments of Colored resin in 264/171, 259, 40,260, 267, 254, 255, 171; a groove formed in a groove formed in a surfaceof a 117/333, 37 R; 118/7, 9, 56; 356/42, 243 transparent body, so thata color strip is formed in which at least one of three color properties,hue, value 5 References Cited and chroma varies continuously along thelength of UNITED STATES PATENTS 3,565,737 2/1971 Lefevre et al 264/171 X7 Claims, 10 Drawing Figures PATENTEU BET 9 73 SHEET 10F 2 (III! PATENTEl] am. 91915 SHEET 2 1F 2 METHOD OF MAKING COMPARATOR COLOR WHEELS Thisis a division of U.S. application Ser. No. 526,532, filed Feb. 10, 1966,and now US. Pat. No. 3,520,626, issued July 14, 1970.

This invention relates to color comparators for evaluating samples takenfor chemical testing, and more particularly concerns an improved colorstandard for such comparators. The invention is particularly, althoughnot exclusively, concerned with providing improved color standards forpH meters of the color comparator type.

Color comparator type pH meters are those intended to give the user amanual reading of the pH of a liquid sample by comparing the color of asample containing a soluble organic dye or indicator" with the color ofa known standard. The color of the indicator'depends on the pH, ornegative logarithm of the hydrogen ion concentration in the sample.

A typical comparator pH meter receives a pair of test tubes or cells anda color standard is arranged to bring various light transmitting, colorshaded windows" adjacent one of the tubes. The sample under test isplaced into both tubes and the pH indicator added to only the tube thatis not seen through the colored window. The standard is then manipulateduntil it appears that both solutions have the same color, and thewindows or portions of the color standard required to equalize thecolors gives a known measure of the color produced by the indicator,that is, the pH of the sample.

Color standards often take the form of wheels havin peripherally spacedwindows colored so that adjacent windows define adjacent steps of thescale used to compare against the test sample and indicator. Such wheelshave been expensive to manufacture, since each window requires accurateand different tinting, and the wheels are capable of giving only rangeapproximations between the steps established by the separate windows.Recently, color standards have become available that use a band ofcontinuously graded color. While these avoid stepwise comparison andinterpolation characteristic of a series of windows, they haveheretofore been available in gradations of a single color, or at most,of two colors. Although these are quite satisfactory for thecolorimetric determination of unknown concentrations other than pH(where generally only one color is involved), they are of limitedusefulness in pH meters for the reason that most indicator dyes changecolor only over a rather narrow range of pH. Consequently, one-color ortwo-color continuously graded color standards for pH measurement areonly useful over the usable range of a single indicator or, at most, forthe range of two indicators.

Accordingly, a primary object of the invention is to provide acontinuously graded color comparator standard useful for a wide range ofpH measurements. A related object is to provide such standards for usein con junction with two, three, or even more different indicators.

Still another object is to provide a transparent color comparatorstandard having a band of continuously graded color which variescontinuously, and in any predetermined manner, with respect to hue,value, and/or chroma. Yet another object is to provide such standard inwhich any number of different hues, values, and/or chromae may bepresent.

Yet another object is to provide a color comparator standard which iseasily manufactured, simple and accurate to use, and stable over longperiods of time.

A further object is to provide a versatile color comparator standardwhich is not limited to two different hues. I

An overall object is to provide novel methods of making such standards.

Other objects and advantages of the invention will become apparent fromthe following detailed description, which is to be read in conjunctionwith the attached drawings in which:

FIG. 1 is an elevation of a pH meter of the color comparator typeembodying the invention;

FIG. 2 is a section taken approximately along the line 2-2 in FIG. 1;

FIG. 3 is an enlarged elevation of the color standard wheel used in thedevice of FIGS. 1 and 2;

FIG. 4 is an end view taken approximately along the line 4-4 in FIG. 3;

FIG. 5 is a perspective of a blank used to manufacture the wheel of FIG.3;

FIG. 6 showsthe blank of FIG. 5 after forming a peripheral flat-bottomedconstant-depth groove of uniform width therein;

FIG. 7 is an enlarged elevation showing a step in one form of themanufacturing process;

FIG. 8 is an enlarged sectional elevation of an apparatus used in themanufacturing process of FIG. 7;

FIG. 9 is an alternative apparatus used in an alternative method ofmaking the color comparator standard; and

FIG. 10 is an enlarged elevation of one part of the apparatus shown inFIG. 9.

While the invention will be described in connection with certainpreferred embodiments, it will be understood that I do not intend tolimit the invention to those embodiments. On the contrary, I intend tocover all alternatives, modifications and equivalents as may be includedwithin the spirit and scope of the invention as defined by the appendedclaims.

Turning first to FIG. 1, there is shown a color comparator l0 embodyingthe invention and comprising a frame 11, having pairs of viewingapertures 12 and 13, and 14 and 15, and a color comparator in the formof a circular disk or wheel 20. In the illustrated construction, theframe 11 includes a box-like container 21, in which the apertures 15, 19are formed and a hinged front door 22, having the apertures l2, l3alined with the apertures 15, 19. The color comparator wheel 20 isfreely pivoted on a post 23, mounted in a block 24 fixed within theframe 1 1, so that the wheel extends laterally through a slot 25 in theside of the frame box 21. The protruding edge of the wheel 20 makes itpossible to rotate the wheel with the box-like frame closed and, byswinging open the door 22, the color comparator wheel 20 can easily belifted from the post 23 and an alternate wheel substituted.

The comparator 10 is also formed with an open topped well 26 adapted toreceive in side-by-side relation a pair of fluid containing cells takingthe form of test tubes 27 and 28.

Pursuant to the invention, the color wheel 20 is formed of transparentmaterial and includes an integral peripheral tinted band 30 ofcontinuously and smoothly varying color. The color will vary, dependingon the requirements of the analysis, with respect to hue (dominantwavelength), value (lightness or saturation) and- /or chroma (purity'orgrayness"); these terms being more fully defined in Kirk & OthmersEncyclopedia of Chemical Technology, Second Edition, Volume 5, pages 801through 812, in the section entitled Color Measurement. A particularfeature of the invention is that band 30 may have increments of two,three, or even more different hues, different values or saturation ofany one or more of each hue, and different chroma or purity of eachcombination of hue and value. In other words, the color comparator ofthe invention is not limited to only one or two different hues, or todifferent values or chromae of a single hue.

Accordingly, in a color comparator standard used for wide range pHmeasurement, the band 30 will vary gradually from red at one extremity31 of band 30 corresponding to a pH of 4., through orange correspondingto pH 5, yellow for pH 6, pea green for pH 7, blue at pH 8, darker blueat pH 9, and violet at pH 10. These hues, with all their nuances ofvalue and chroma, correspond to a wide range, three component, organicindicator dye composed of methyl red (red at pH 4.2, yellow at pH 6.4),brom thymol blue (yellow at pH 6, blue at a pH of about 8), andphenolphthalein (colorless at pH 8.3, pink at pH 10).

The wheel 20 also includes a central hole 33, adapted to fit easily overthe comparator post 23 (FIG. 1), and a scale 34 running along the band30 whose indicia may be viewed through an aperture 35 (FIG. 1) formed inthe door 22 of comparator 10. These indicia correspond with apredetermined color relationship between the color on any point of band30 and the concentration of unknown in the sample undergoing test. Anapproximate calibration may be accomplished by appropriate design ofcolor wheel 20 and appropriate formulation of colored material in band30, as will appear hereinafter, or may be made exact by calibrationagainst solutions of known composition.

To operate the comparator 10, a fixed amount of the solution to betested is placed in each of cells or tubes 27 and 28, and an indicatoris added only to tube 28. For a wide range pH measurement, a mixture ofpH indicators is used. The indicator produces a color change in thesolution in tube 28, with the hue, value, and chroma being dependent onthe composition of the original test solution in tube 28 and on theamount and composition of indicator. Since indicator concentration isheld constant from test to test, the principal color variable will beits hue, while value and chroma will be relatively unaffected.

The comparator 10 is then held to the light so that the user can viewapertures 12 and 13 (FIG. 1). The color seen through aperture 13 (andaperture 15 in FIG. 2) has a hue which is dependent on the pH of thetest sample contained in tube 28. That portion of wheel betweenapertures 13 and 15 is completely transparent.

Color wheel 20 is provided with a tinted band 30 having variations inhue, value, and chroma appropriate for the particular indicator beingused. Hence the color seen through the apertures 12 and 19 (FIG. 2) isthat of a small portion of the peripheral color band 30 as modified bylight passing through the tube 27 which contains the sample under testbut without a pH indicator added.

The wheel 20 is then turned by manipulating the projecting portion atthe right-hand side of the comparator 10 until, to the observer, thecolor appears identical at the apertures 12, 13. The angular position ofthe wheel when this occurs is thus indicative of the point along thecolor band 30 which matches the extent of the color change in the tube28. Thus, by standardizing the color band of the wheel 20, a reading ofthe scale 34 through the aperture 35 gives a direct indication of the pHin the sample under test. It will be appreciated that the factorsaffecting the color seen at the apertures l2, 13 are identical exceptthat the color at the aperture 12 is affected by light transmittedthrough the color band 30 and the color seen at the aperture 13 isaffected by light transmitted through the pH indicator containingsolution in the tube 28.

Referring to FIGS. 3 and 4, the color wheel 20 is a transparent body inthe shape of a flat disk (FIG. 5) having an integral band or strip 30 ofsmoothly and continuously varying hue, value, and/0r chroma. Strip 30 isannular, extending about the periphery of the wheel 20 for substantiallyall of its circumference, and it is of a substantially uniform widthmeasured radially of the wheel.

As may best be seen with reference to FIG. 6, the band or strip 30occupies, and is defined by, an annular groove 41 that has a flatbottom, is of constant depth, and is of uniform width. The materialmaking up the strip 35 is carefully prepared in accordance with theinvention so that it presents smoothly varying changes in hue, value,and/or chroma as the strip is viewed perpendicularly through the planeof the wheel 20.

In making a transparent color comparator standard, a blank 40 (P16. 5)of a transparent organic'resin is first cut to the desired shape, and acentral pivot hole 33 is drilled therein. Thereafter, an annular groove41 (FIG. 6) is milled in blank 40 to a constant depth and width, and thebottom surface of groove 41 is polished flat and smooth. Alternatively,groove 41 may be formed in the disk or blank 40 by casting the blank ina mold having a shape corresponding to groove 41. Similarly, pivot hole33 may be formed during the molding operation.

The colored strip 35 may advantageously be formed by either of twomethods, both to be described hereinafter. Broadly, in the first methodtwo or more batches of a viscous, hardenable, transparent organic resin,each tinted to an appropriate predetermined color (i.e., combination ofhue, value, and/or chroma) are prepared, and an increment of each resinis deposited in a portion of groove 41. The increments are thereaftercarefully mixed by a mixer that moves longitudinally along groove 41 soas to form a strip of smoothly and constantly varying color. The resinin strip 35 is thereafter hardened, and is shaped, as for example byplaning and polishing.

In the second method, two or more different colored resins are preparedin the manner of the first method,

but instead of depositing increments of each separate resin in groove 41the different resins are pumped as continuous streams, with the separatestreams being blended in continuously varying proportions, so as toproduce a single mixed stream of continuously varying color. Thislast-named stream is delivered at a constant flowrate, and is depositedin groove 41, the stream flow being synchronized with movement of thegrooved blank 40. As before, the resin is thereafter hardened and shapedto provide a constant depth band 35.

The color wheel blank 40 (FIG. 5) is advantageously made of atransparent synthetic organic resin that is compatible with the resinused for band 35 (FIG. 3), so that the two form a monolithic,inseparable structure. The acrylic resins, particularlymethylmethacrylate, are particularly preferred by reason of theirtransparency and their excellent mechanical properties. Blank 40typically has dimensions of 3% inches in diameter, onefourth inch thick,and is provided with a groove 41 that is about one-eighth inch deep andfive-eighths inch wide.

Irrespective of whether the incremental pouring or the continuouspouring technique is used to prepare color wheel 20, it is firstnecessary to formulate a solution of casting resin dyes which, whenmixed with the casting resin, will have a color corresponding with asample solution of known pH containing an indicator.

The initial step in formulating colored casting resins is thepreparation of liquid samples of known pH, containing a constant andpredetermined amount of one or more indicators. For purposes ofillustration, instructions will be given for preparing a colorcomparator standard for use with brome thymol blue pH indicator,although it will be appreciated that the same technique may be employedwith other indicators or with mixtures of two or more indicators, asdiscussed earlier.

Two samples of known pH are then prepared, for example one at pH 6.0when the brome thymol blue indicator is yellow and the other at pH 8.5when it is a deep blue. These solutions are separately analyzed by aspectrophotometer, and absorption curves are made over the entirespectrum. Then, by trial and error, a standard dye, or mixture ofstandard dyes, from among the commercially available casting resin dyesis selected to produce a casting resin solution whose absorption curveclosely matches the absorption curve of each of the two known samples.This is most easily done by preparing beforehand absorption curves ofselected concentrations of a great number of suitable dyes. By reviewingthe file of curves, it is a relatively simple matter to select the dyeor dyes most likely to produce the desired color.

In the example under discussion, it is found that the color of a bromethymol blue solution at pH 6.0, viewed through a LG inch tube, can beclosely color matched (as to hue, value, and chroma) by a /3 inchthickness of methylmethacrylate casting resin containing 0.0082 percentof lrgacet Yellow GL and 0.000064 percent of Solvent Rubine 3 D. At pH8.5, the brome thymol blue indicator has a color which, through a 1.0inch tube, corresponds with a :6 inch thickness of methylmethacrylateresin containing 0.006 percent Oil Fast Blue R and 0.0048 percent oflrgacet Brilliant Blue 2 GLN.

Each of the resins is separately formulated by adding the respectivedyes and'a polymerization catalyst to methylmethacrylate monomer, andheating the mixtures until partial polymerization occurs to an extentsuch that the solutions are syrupy yet remain sufficiently fluid thatthey may be poured or pumped. Various catalysts are useful for thepolymerization, and the peroxide catalysts such as benzoyl peroxide orthe azo catalysts such as azobisisobutyronitrile may be used, theconcentration in either case being about 0.5 weight percent on resin.

The several resin compositions used in a given color wheel will ofcourse depend on the type of unknown undergoing test and on the type ofindicator used. It is however necessary to provide at least two resinsof different composition, that is, color, although in particularcircumstances one of the resins may be colorless. Accordingly, the termcolor, as used herein, unless otherwise stated, is intended to embrace acompletely transparent resin. An example of the use of two resins whereone is transparent is in preparing a disk for the colorimetric analysisof water for chlorine using an orthotolidine indicator; in this case aconcentration of 1.0 ppm. chlorine in a 1.0 inch test tube will have thesame color as a Va inch thickness of methylmethacrylate containing0.0074 percent Amplast Yellow G, while a null concentration will be astransparent as uncolored polymethylmethacrylate.

As earlier stated, two different methods of manufacture may be used toproduce the color wheels of the invention. The first method, as depictedin FIGS. 7 and 8, entails the depositing of discrete increments of twoor more different colored resins in groove 41 (FIG. 6), followed bymixing the increments to form a band 30 of continuously varying color.As typified by FIG. 7, five batches, 35a through 354:, of differentlycolored resins are deposited at spaced locations in groove 41; five suchresins are needed for a wide range pH indicator suitable for pH readingsbetween 4 and 10. The five resins have each been colored to correspondwith the hue, value, and chroma of five pro-selected samples of knownpH, one at pH 4.0, another at pH 10.0, and the remaining three atspaced-apart pHs between 4.0 and I0.0. Each of the resins is a viscouspartially polymerized solution of polymethylmethacrylate, which has beencooled after partial polymerization to halt further polymerization.

Turning now to FIG. 8, the blank 40 is placed face up on a turntable,not shown, and mixer 60 is lowered into position with its blades 64 and65 within groove 41 and immersed in the syrupy resin. It will beobserved from this figure that mixer 60 is provided with a pair ofcounter-rotating shafts 66 and 68, each terminating in arm 6i and 62,respectively, at the ends of which are the blades 64 and 65. The axes ofshafts 66 and 68 are in a plane aligned along a radius of blank 40 sothat more of the mixing occurs by movement of the resin in a directiontransverse to the length of groove 41 than in a direction longitudinalof the groove.

Counter-rotation of shafts 66 and 68 is accomplished by providing a pairof gears 69 and 70 mounted on the shafts, and by driving only one of theshafts, specifically, 66. Gears 69 and 70 are enclosed wihin housing 71.

Mixer 60 is then moved along groove 41, or vice versa, in order toeffect mixing between adjacent batches 35a-35e (FIG. 7) and thereby forma band of continuously varying color.

Alternatively, the system of FIGS. 9 and 10 may be employed to provide aband of continuously varying color. In this embodiment five batches ofresins of different color are prepared as before, but instead of addingthe batches as separate increments to groove 41 of block 40, the resinsare formed into streams and blended continuously, but in continuouslyvarying proportions, and the resultant stream poured into the groove.

Referring to FIG. 9, blank 40 of color wheel 20 is placed on turntable72, and is centered with a central pin 74 passing through pivot hole 33.A series of three clamps, shown schematically at 75, blocks color wheel20 onto the turntable, which is driven via shaft 76 coupled viaschematic shaft 77 to motor M. Motor M also drives, in snchronization,schematic shaft 79 which in turn is coupled to cam shaft 79.

Each of the five colored casting resins is transferred from a tank, notshown, via conduit 80 and check valve 81 to one of the dam-driven pistonpumps 82.

Pumps 82 each comprise an external cylinder 84 and a spring-biasedpiston 85 which is driven inwardly of the cylinder 84 by cam 86 fixed tocam shaft 79.

The discharge from pump 82 flows through conduit 88, check valve 89,solenoid valve 90, and conduit 91 to mixing head 92, which will bedescribed peres ently.

Returning to piston pump 82, it was noted that each pump is providedwith a separate cam 86. These cams are so designed that they willdeliver varying amounts of resin from each of the pumps 82, 82b, 82c,82d, and 82a, but the total flowrate from all of the pumps is constant.(For simplicity, the connecting piping from each of the pumps 82bthrough 82c extending to mixing head 92 has been omitted from thedrawing. It will be understood however that each of the respective pumpdischarge lines communicates with mixing head 92).

The streams from each of the pumps 82 through 82a commingle in mixinghead 92, best shown in FIG. 10. There it is seen that conduits 91, 91c,etc. are threadedly received into threaded ports 93 leading to a mixingchamber 95 having cylindrical sides and a frustoconical lower portion,and wherein rotary paddle mixer 96 is disposed. Mixer 96 is connectedvia shaft 98 to an external motor, not shown. The mixer itself has twoeccentric blades or paddles, 99 and 100 to effect rapid mixing of theviscous resin streams entering through conduits 91, 91c, etc., and fordischarging them rapidly as a single mixed stream via conduit 101located at the bottom of the lower frustroconicai portion of mixingchamber 95. For repair or replacement, mixer 92 may be disassembled byremoving the top portion 302, normally secured to the lower portion 104via cap screws, not shown.

Returning to FIG. 9, the mixed stream from conduit is deposited intogroove 41 of block 40 while turntable 72 rotates in synchronization withcam shaft 79. Thus, one rotation of turntable 72 corresponds to onecomplete cycle of cam shaft 79, during which the several resins areproportionately mixed and discharged to fill the groove 41.

Whether made by the method and apparatus indicated in FIGS. 7 and 8 orthose in FIGS. 9 and 10, it

is then necessary to harden the colored resins, which is ordinarilyaccomplished by heating the color wheel to a sufficient temperature topermit the polymerization reaction to proceed to completion. The wheelor disk is then cooled and the top surface polished flat.

It will be readily apparent that the color comparator 10 is aneconomical and easily used instrument. The color wheel 20 may bemanufactured easily and reproducibly, particularly once the proper dyeconcentrations have been determined. The smoothly graded color bandsproduce an exception reading accuracy, normally within a few percent ofthe full scale of a wheel. This of course is in substantial contrast tothe accuracies achieved by comparators having spaced windows, andapproaches the accuracy of expensive electric or electronic pI-I meters.Furthermore, interpolation is relatively simple inasmuch as peripheraldistances along the band are approximately linear with pH.

I claim as my invention:

1. The method of making a transparent colorcomparator standard having atransparent strip of at least one gradated color property selected fromthe group consisting of hue, value and chroma, which comprises: formingwithin a flat-bottomed constant depth groove of uniform width located ina transparent body, a constant depth strip having at least said oneproperty that varies continuously along said groove, by depositingwithin said groove viscous, hardenabie, transparent organic resincompositions having at least said one property in varying degrees saidresin being compatible with the material of said body and forming amonolithic inseparable structure therewith, hardening said depositedresin and shaping said deposited resin to form said strip.

2. The method of claim 1 including the step of calibrating said standardwith respect to said color property.

-3. The method of claim 1 including depositing said resin as acontinuous stream at a constant flow rate, the composition being variedcontinuously as to said color property, along the length of said groove.

4. The method of claim 1 wherein said resin is deposited as a pluralityof contiguous discrete increments along said groove, each said incrementvarying as to said color property from the preceding increment, moving amixing means along said groove to mix the increments of resin to formsaid strip having said gradated color property.

5. The method of claim 1 wherein said body is a circular disk oftransparent organic resin compatible with said viscous hardenabletransparent organic resin, said groove is along the periphery of oneface of said disk, and said disk has a central pivot hole.

6. The method of claim 1 wherein said resin as a partially polymerizedmonomer.

7. The method of making a color-comparator standard for indicating theconcentration of a specific substance in a cell of fixed viewingthickness comprising: forming a body of transparent material havingtherein a flat-bottomed constant depth groove of uniform width; making aplurality of spectre-photometric absorption curves from a plurality ofknown concentrations of substance to which a color indicator has beenadded; mixing a plurality of solutions of casting resin dyes torespectively correspond in at least one color property to points on eachof said spectrophotometric absorption curves; tinting a plurality ofhardenable, transparent organic resins with said solutions; depositingthe tinted resins in said groove to form a transparent stripcharacterized by having said color property in continuously varyingdegree along the length thereof so that the varying color property ofsaid strip is related to varying concentrations of said substance, andhardening said resin.

2. The method of claim 1 including the step of calibrating said standardwith respect to said color property.
 3. The method of claim 1 includingdepositing said resin as a continuous stream at a constant flow rate,the composition being varied continuously as to said color property,along the length of said groove.
 4. The method of claim 1 wherein saidresin is deposited as a plurality of contiguous discrete incrementsalong said groove, each said increment varying as to said color propertyfrom the preceding increment, moving a mixing means along said groove tomix the increments of resin to form said strip having said gradatedcolor property.
 5. The method of claim 1 wherein said body is a circulardisk of transparent organic resin compatible with said viscoushardenable transparent organic resin, said groove is along the peripheryof one face of said disk, and said disk has a central pivot hole.
 6. Themethod of claim 1 wherein said resin is a partially polymerized monomer.7. The method of making a color-comparator standard for indicating theconcentration of a specific substance in a cell of fixed viewingthickness comprising: forming a body of transparent material havingtherein a flat-bottomed constant depth groove of uniform width; making aplurality of spectro-photometric absorption curves from a plurality ofknown concentrations of substance to which a color indicator has beenadded; mixing a plurality of solutions of casting resin dyes torespectively correspond in at least one color property to points on eachof said spectrophotometric absorption curves; tinting a plurality ofhardenable, transparent organic resins with said solutions; depositingthe tinted resins in said groove to form a transparent stripcharacterized by having said color property in continuously varyingdegree along the length thereOf so that the varying color property ofsaid strip is related to varying concentrations of said substance, andhardening said resin.